Embryology. 



121 



the zona pelhicida, which contains — first, the proto- 

 plasmic cell-substance or "yolk," within which lies, 

 second, the nucleus or germinal vesicle, within which 

 again lies, third, the nucleolus or germinal spot. This 

 description is true of the egg-cells of all animals, 

 if we add that in the case of the lowest animals — such 

 as sponges, &c. — there is no enveloping membrane : 

 the egg-cell is here a naked cell, and its constituent 

 protoplasm, being thus unconfined, is free to perform 

 protoplasmic movements which it does after the 



Fig. 30. — Ovarian OTum of a Mammal, \a) magnified and viewed under 

 pressure, (J)) burst by increased pressure, with yolk and nucleus 

 escaping : (r) the nucleus more freed from yolk-substance. (From 

 Quaui's Anatomy, after Allen Thomson.) 



manner, and with all the activity, of an amoeba. 

 But even with respect to this matter of an enveloping 

 membrane, there is no essential difference between 

 an ovum of the lowest and an ovum of the highest 

 animals. For in their early stages of development 

 within the ovary t'ne ova of the highest animals 

 are likewise in the condition of naked cells, exhibiting 

 amoebiform movements ; the enveloping membrane 

 of an ovum being the product of a later development. 



